There exists a need for a better, more adaptable, and particularly a faster method for joining metals. Presently, the two most widely used methods are soldering and welding.
In any of the well known soldering or brazing processes the joint between the two metal parts, consisting of either the same or different metals, is made by means of another metal which has a melting point well below that of the metals to be joined. Adhesion of the solder-metal to the solid surfaces of the parts to be joined is assured by various means, e.g. the removal of oxide layers by means of fluxes, by means of mechanical abrazion, or by both. The solder metal adheres to the parts' surfaces through physical adhesion (van-der-Waals forces between the atoms and molecules). Solder metal atoms may also diffuse into the solid metal to a depth of a few atomic layers; yet, only the solder metal is ever melted in the process, the parts to be joined, including their surface, remain solid at all times. These processes have also been called "Liquid-Solid Phase Joining", for instance in the book by George E. Linnert "Welding Metallurgy" (3rd edition, 1965, The American Welding Society, New York).
In a related type of process, surfaces of one type of metal are being covered with another type of metal, or with metal oxides and carbides, by spraying them against the surface in the form of heated or sometimes melted fine particles. They may be heated and fed through a oxyfuel-gas flame, the detonation of an oxyfuel gas mixture, or an electric arc. Trade names used for this process are "metallizing", "flame spraying" and "plasma plating". The surfaces to be coated are not melted. The process is so slow that, if enough heat were supplied to melt the surface, the whole workpiece would melt. At the impact point of the particles there may be, however, some locallized melting.
Surfaces have also been altered by locallized heating and melting under a high power density laser beam or electron beam. The rapid self-quenching which a thin heat-zone experiences can lead to a hardening effect due to phase changes (in the solid metal). The particles on a flame-sprayed surface have also been re-melted, in order to fuse or braze them to the underlying solid surface; a brazing flux is sometimes mixed-in with the particles when they are sprayed on. Compared with these practices, the novel process described here, even when it is used for surface cladding (i.e. joining a surface layer of one metal onto another metal), is decidedly different, insofar as the original surface of the bulk metal part is everywhere truely melted in the process.
When joining two metal parts by fusion welding the adjacent regions of these parts themselves are being melted, the melt then forming one single melt-puddle, which subsequently solidifies, thus producing a solid metal connection between the parts. The numerous methods which can be applied to weld in the just indicated manner, include, for instance, a flame, an electric arc, a corpuscular beam such as an electron beam, a light beam such as a laser beam, resistive heating by an electric current, heating and melting by friction or by ultrasonic vibrations.
The term welding is also applied to some solid to solid bonding methods, which all apply pressure, usually by mechanical means, sometimes by the use of explosives. In the present context we don't need to discuss these methods.
Soldering as well as welding have their specific advantages and disadvantages. A solder joint is usually weaker than a weld, but it permits greater mechanical precision to be maintained, there are less thermal stresses and distortions. For soldering, the parts must be fitted precisely, although solder metal can be used to fill gaps. The joint is then usually weak. In welding, gaps between the parts can be filled by melting down a sufficient amount of "filler" or "welding wire" or "welding rod", which consists usually of a metal alloy closely similar to the metal of the parts which are to be joint. It is supplied as solid wire or rod. When welding thick pieses, a pre-machined V-groove is filled up by depositing successive "ropes" of metal from a welding rod or wire, one rope being layed on top of the other. The filler or welding rod is melted at the same time and together with a locallized region of the work piece. Any of the heating methods used in autogenous welding (i.e. without addition of other metal) which have been mentioned above, can be used for welding with a filler as well.
The fact that the welding rod and a part of the work piece must be melted at the same time, for instance by the same electric arc, poses certain difficulties, one of them being that a great amount of heat flows into the work piece leading to thermal stresses and distortions. The newer processes of welding with a laser or electron beam minimise the heat input because of the great power density which they make available and which permits locallised melting of the work piece before much energy has been conducted into the interior of the piece by heat conduction. The conditions which must be met to achieve this kind of "quasi-adiabatic" melting of a surface layer have recently been analyzed and described by me. (Paper entitled: "Quasi-adiabatic melting and vaporization due to a radiation beam of high power density" published in OPTIK, vol. 39, 1974, p. 558-580; in English). Even when welding with such high power-density radiation beams is it possible to feed a welding wire into the beam or into the beam generated melt-puddle in order to fill up gaps between the pieces to be joined. However, since the process operates with a minimum of excess energy, yet differences in the width of the gaps require different amounts of metal to be deposited, the control of the process becomes very difficult when filler wire is required. In order to maintain the conditions for minimum heat conduction losses, the melting of the surface must proceed extemely rapidly, and no time and energy must be waisted in melting a filler wire, or the advantages of the beam welding processes are lost. The above are obviously partly incompatible requirements. The invention described here later shows how this incompatibility can be overcome.
Although the electron beam, and to a lesser extend the laser beam, can make single-pass welds in fairly thick plates (up to a thickness of 5 cm with a 60 kW electron beam in air, up to 30 cm in vacuum) the energy input still takes place on a surface, in this case the surface of what has been called a "key hole" or a "crater" which penetrates the work piece to the thickness of the weld. A mathematical analysis of some aspects of this process has been provided by J. H. Fink (Welding Research Supplement to the Welding Journal, May 1975, pages 137s-153s). Thus it is quite justified to speak of a radiation beam as being essentially a surface heating source, as we do in the following, and although its small but finite depth of penetration into the surface is of great significance (see OPTIK 39,558, quoted above).
The new process, described below, utilises the inherent, special capabilities of these high power-density radiation beams, namely their capability to melt a thin surface layer only to the fullest extend. No attempt is made to fuse the parts, based on this energy source alone. Thus, the new process permits joining at speeds equal to or exceeding those of conventional electron beam or laser beam welding--and it retains the advantage of minimum heat input into the work piece.